The objective of our studies on virus and membrane structures is to analyze the assembly and stability of macromolecular aggregates which exemplify the processes of normal and pathological growth and differentiation. The organization of the nucleohistone in polyoma virus is being studied by X-ray cyrstallographic analysis of virus and capsid crystals. The stability and hydration of several small viruses are being characterized by X-ray scattering measurements from solutions. Structural transitions in the spirally wrapped myelin membrane array are being investigated by X-ray diffraction and electron microscopy. The structural basis of electrical and chemical connections between cells is being studied by X-ray diffraction and electron microscopy of purified gap junctions from mouse heart and liver. New X-ray diffraction techniques are being developed to explore the molecular dynamics and structure of viruses and membrane assemblies.